A result meant to be held onto and iterated — code, a document, a diagram — is awkward to live inside a chat bubble. You have to scroll to find it, and each revision stacking as a new bubble makes it unclear which one is current. A canvas artifact pulls that result out of the conversation into its own panel, leaving the chat to carry just the instructions about it — "make paragraph 3 shorter."
The panel slides in from the side when needed, and its content updates in place with each new version rather than piling up new chat bubbles. A way to step back to an earlier version makes it much less risky to experiment.
On narrow screens there's no room to show both side by side, so it collapses into a tab that switches between the conversation and the panel.
When to use
Use it for something iterated on repeatedly — code, a long document, a diagram. A short answer read once isn't worth opening a panel for.